Fuel dispensers are equipped with a unit for measuring the volume of fuel dispensed. This unit, also known as a measurer, is generally a volumetric meter and can be either a mechanical device or a static device.
The function of a mechanical measurer is to convert the flow of the fuel into rotary motion in which one complete revolution corresponds to a given volume of fuel passing through the measurer. An optical or magnetic encoding system responsive to the rotary motion supplies an electrical signal made up of a series of pulses each of which corresponds to a volume measurement increment, for example 1 centiliter (cl).
Static measurers have no moving parts. They include ultrasonic measurers and fluidic oscillators.
With some measurers, in particular fluidic oscillators, the resulting volume measurement can be influenced by the viscosity of the fluid, and to be more precise by its Reynolds number, which operates in the form of an error function relative to a main function that is independent of the Reynolds number. This applies in particular to fluidic oscillators, in which the frequency of oscillation is proportional to the flowrate of the fluid to a first approximation. However, a correctional term dependent on Reynolds number must be taken into account if sufficient accuracy is to be achieved.
It should be emphasized that the viscosity of a fluid is not constant, and that it can vary for the same fluid from one shipment to another. In the case of fuel, the viscosity can change during a single dispensing operation because of temperature fluctuations. If sufficient measuring accuracy is to be achieved, it is sometimes essential to know in real time the viscosity or the Reynolds number of the fluid for which the dispensed volume is to be measured, these two magnitudes being proportional.
Various methods are known for measuring the viscosity of a fluid.
The Couette method measures the force mechanically resisting the movement of two plates relative to each other due to the displacement of a thin film of fluid contained between the two plates. The dynamic viscosity .mu. of the fluid is directly proportional to the measured resisting force. In practice, the plates are usually in the form of two coaxial cylinders and the viscosity is determined by measuring the torque needed to immobilize one of the two cylinders when the other is rotating at a given speed. That method, which is widely used, has the advantage of allowing continuous measurement. On the other hand, the devices employed are complex and fragile (fine guides, presence of a motor, a torque meter, etc.).
Other methods are based on Poiseuille's law which states that, for laminar flow in a capillary tube, the flowrate and the head loss between the ends of the tube are proportional, the coefficient of proportionality depending on the viscosity of the fluid. There are many devices operating in accordance with that principle. The simplest of them includes a vertical capillary tube having a central enlargement. The viscosity of the fluid is measured by measuring the time required for the fluid to flow under its own weight between two marks at opposite sides of the enlargement. The drawbacks of that type of technique are as follows:
measurement is non-continuous, and can only be effected by periodically taking samples of the fluid, PA1 manual intervention is necessary, PA1 each tube provides only a small range of measurements. PA1 capillary tubes of very small diameter can easily become blocked.
The diameter of the capillary tube is chosen to maintain laminar flow, i.e. a Reynolds number of low value. Because Reynolds number Re is dependent on the dynamic viscosity .mu. of the fluid (Re=4.rho.Q/.pi..mu.d with .rho.=density of the fluid, Q=flowrate, d=tube diameter), the same tube can be used for only a limited range of viscosities, and
Finally, it may be observed that the two methods described hereinabove can only be used at low flowrates Q of less than 1 liter per minute (1/min) if laminar conditions are to be maintained.